page_title: Niobium
section_title: Halides
hierarchical_section_title: Niobium / Compounds / Halides
caption_attribution_description: Ball-and-stick model of the niobium pentachloride molecule, Nb2Cl10. X-ray crystallographic data from Acta Cryst. (1991). C47, 2435-2437.
context_page_description: Niobium, also known as columbium, is a chemical element with the symbol Nb and atomic number 41. Niobium is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to that of pure titanium, and it has similar ductility to iron. Niobium oxidizes in the earth's atmosphere very slowly, hence its application in jewelry as a hypoallergenic alternative to nickel. Niobium is often found in the minerals pyrochlore and columbite, hence the former name "columbium". Its name comes from Greek mythology, specifically Niobe, who was the daughter of Tantalus, the namesake of tantalum. The name reflects the great similarity between the two elements in their physical and chemical properties, making them difficult to distinguish.
  The English chemist Charles Hatchett reported a new element similar to tantalum in 1801 and named it columbium. In 1809, the English chemist William Hyde Wollaston wrongly concluded that tantalum and columbium were identical. The German chemist Heinrich Rose determined in 1846 that tantalum ores contain a second element, which he named niobium.
context_section_description: Niobium forms halides in the oxidation states of +5 and +4 as well as diverse substoichiometric compounds. The pentahalides (NbX
  ₅) feature octahedral Nb centres. Niobium pentafluoride (NbF₅) is a white solid with a melting point of 79.0 °C and niobium pentachloride (NbCl₅) is yellow (see image at left) with a melting point of 203.4 °C. Both are hydrolyzed to give oxides and oxyhalides, such as NbOCl₃. The pentachloride is a versatile reagent used to generate the organometallic compounds, such as niobocene dichloride ((C
  ₅H
  ₅)
  ₂NbCl
  ₂). The tetrahalides (NbX
  ₄) are dark-coloured polymers with Nb-Nb bonds; for example, the black hygroscopic niobium tetrafluoride (NbF₄) and brown niobium tetrachloride (NbCl₄).
  Anionic halide compounds of niobium are well known, owing in part to the Lewis acidity of the pentahalides. The most important is [NbF₇]²⁻, an intermediate in the separation of Nb and Ta from the ores. This heptafluoride tends to form the oxopentafluoride more readily than does the tantalum compound. Other halide complexes include octahedral [NbCl₆]⁻:
  Nb₂Cl₁₀ + 2 Cl⁻ → 2 [NbCl₆]⁻
  As with other metals with low atomic numbers, a variety of reduced halide cluster ions is known, the prime example being [Nb₆Cl₁₈]⁴⁻.
caption: Ball-and-stick model of niobium pentachloride, which exists as a dimer
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